There are a substantial number of machines which perform operations on a tensionally fragile web of material. For example, in U.S. Pat. No. 3,802,942, to Amberg et al., there is disclosed a machine for forming a pre-printed web of foam plastic material into a plurality of successive cylinders which are then applied around glass or plastic containers and subsequently shrunk into conforming engagement with such containers. The foam plastic material, which may be either foam polystyrene or a foamed polyethylene, does not have significant tension resistance and, since the machine operates by pulling the web of material from a supply roll, it is essential that the tension in the web be at all times maintained within a desired control range. Upon the depletion of one supply roll, it is customary to splice the end of the web to the beginning end of the web mounted on the new supply roll. This operation inherently would result in a very substantial increase in web tension, since the new supply roll is essentially stationary and must be accelerated by the web so that the linear speed of the web being drawn from the new supply roll corresponds to the operating speed of the machine.
Prior mechanisms for controlling web tension have involved the engagement of a shiftable festooning roller with the web. A substantially constant fluid pressure bias is imposed on the festooning roll in opposition to the web tension forces extended on the roller so that the roll shifts in response to variations in the web tension. Such shifting movements of the festooning roller are utilized to operate a hydralic valve unit which varies the amount of drag force applied to the supply reel. This general type of speed control arrangement is described in U.S. Pat. No. 3,862,724 to John R. Johnson et al.
When a single festooning roller is employed for the web tension control, it necessarily follows that the tension control range must be relatively broad so as to encompass not only the tension variation encountered during normal operation of the processing machine, but also to accomodate the very significant increase in web tension that could occur when the end of the web is spliced onto a new supply roll. Inherently, the requirement that a single roller accommodate such a wide range of potential web tension results in the web tension control not being particularly precise in the operating range. There is, therefore, a distinct need for a web tension control for tensionally fragile web material which will precisely maintain a desired control range of tension for normal operation conditions, and yet will accommodate the substantially increased tension that would otherwise result from the splicing operating when the end of the web is transferred for an empty roll to a fresh supply roll.